The Belle II experiment and SuperKEKB: adddressing questions about the Cosmos

The particle accelerator SuperKEKB, in Japan will break the world record of particle accelerator luminosity and particle collision rate

The Spanish contributors IFIC, IFCA and ITAINNOVA have developed a new detector for Belle II: DEPFET

These joint efforts will contribute data to answer fundamental questions about the Cosmos

A few words about elementary particle physics

In particle physics, an elementary particle is one without any further sub-structure. The particles currently considered to be elementary are the fermions and bosons. Of those, fermions include quarks, leptons, antiquarks and antileptons, and are considered to be the constituent elements of matter and antimatter. Bosons include gauge bosons and the notorious Higgs boson, and are considered to be “force particles”, mediating interactions between fermions. Collisions between elementary particles by means of particle accelerators constitute invaluable means of studying these particles, and more widely, matter and the Universe.

The Standard Model is a theory in particle physics, which describes the elementary particles composing the invisible matter of the Universe and their interactions. It describes three of the four known fundamental forces operating in the Universe: the electromagnetic, weak, and strong interactions (hence, gravitational forces are not included).

Diagram of interactions between elementary particles.

The Belle II experiment was launched in the pursuit of data to support the elaboration of the theory beyond the Standard Model, Belle II will, to that aim, be measuring unusual disintegrations of elementary particles, corpuscles displaying names as curious to the non-specialist as the tau leptons (particles related to electrons), the “beauty quark” or the “charm quark”.

During its time in operation, Belle II will be looking for evidence of new particles helping to solve pressing questions about the Cosmos. For instance: why is the Universe dominated by matter and not by antimatter, if they must have been formed in equal amounts after the Big Bang?

SuperKEKB and Belle II

SuperKEKB is a particle accelerator with a perimeter of 3 km, located at the facilities of the High Energy Accelerator Research Organization in Tsukuba (prefecture of Ibaraki, Japan). It consists of two rings: one for the high-energy electron beam (HER), and one of lower energy for the positron beam (LER). It is a “B Meson Factory”, specialized in producing these particles.

Unlike the largest, most powerful, proton accelerator of the World, the Large Hadron Collider of CERN, in Geneva (Switzerland), the SuperKEKB is designed to be the accelerator with the strongest luminosity. Luminosity is a measure of the number of potential collisions in an accelerator per unit of surface over a given time period. SuperKEKB leads the way moving the so-called “frontier of luminosity” forward. It will expectedly break the current World Record of luminosity held by its predecessor KEKB since year 2009.

Belle II is, in essence, a detector module located at one of the four experimental zones of the SuperKEKB accelerator. It has a 10x10 metre height and width, and a weight of 1,500 tonnes and is equipped with a set of different subdetectors. Those detectors, located around the tube where collisions take place in SuperKEKB, take measurements to characterise and identify the particles formed from collisions.

The newly operating SuperKEKB accelerator and the Belle II detector module, in Tsukuba (Japan), are designed to search for a new physics theory, going further ahead of the Standard Model. Spanish participants have contributed to the design, construction, installation and operation of DEPFET, a new detector for the Belle II experiment.

Start of the Belle II experiment

The first electron beam was stored in the main high-energy ring of the SuperKEKB accelerator on March the 21st, 2018. Ten days later, on March the 31st the first positron beam was stored. Adjustment and fine-tuning followed to ensure beam collision at the centre of the Belle II detector, before any data collection would take place.

Electrons and their counterpart antiparticles, positrons, were accelerated and accumulated by the SuperKEKB accelerator. They were collided on past April the 26th at 00:38, with the first collisions registered in the accelerator of the High Energy Accelerator Research Organization (KEK) in Japan since its predecessor, KEKB, finished operation in 2010.

Belle II registered the mutual annihilation of particle and anti-particle, the collision of these electrons and positrons generating new matter elements: quark pairs and “b” quarks and antiquarks (“b” for “bottom” or “beauty”), of which the latter are amongst the heaviest of all quarks.

The project

An international team of collaborators from more than 750 researchers from 25 countries designed and built the Belle II detector module. It has enormously improved capabilities over its predecessor, and is able to detect and reconstruct events far faster. It does so by leveraging the 40-fold higher luminosity of SuperKEKB if compared to KEKB.

In the Spanish contribution to Belle II, a new detector was developed, called DEPFET. The Spanish partnership joined the expertise and know-how of the Institute for Corpuscular Physics (IFIC, joint institute of CSIC and Universitat de València), the Instituto de Física de Cantabria (IFCA, centro mixto CSIC-Universidad de Cantabria), the Instituto Tecnológico de Aragón (ITAINNOVA) and the Instituto Nacional de Técnica Aeroespacial (INTA). In addition, international collaborating partners encompass the Max Planck Institute of Physics in Munich, the Vienna Institute of High-energy Physics (HEPHY) and the European project AIDA2020 on Advanced Infrastructures for Accelerators and Detectors.

The newly developed DEPFET active pixel detectors are composed of a single silicon piece integrating the sensor with the physical support, onto which reading chips are welded. The result is a pixel detector of extreme thinness (75 microns at the active area). This reduces the alteration of particle trajectory resulting from the interaction with detector materials. IFCA and INTA contributed to the in-built Pixel Vertex Detector (PXD), a component designed to collect information about particles coming from the collision location. ITAINNOVA contributed towards keeping electromagnetic noise (EMC) under control, allowing to ensure correct functioning of experiment electrical systems.

Different components of the Belle II detector module.

IFIC has been for more than a decade one of the participants in the development of DEPFET. Firstly, coordinating the tests with particle beams, and later on, working on the detector refrigeration strategy, result of the Ph.D. thesis of Carlos Mariñas. IFIC has, moreover, designed and produced the electronics to check the correct working of the different modules of DEPFET once assembled on Belle II.

Hear the voices of some participants

To the director of KEK, Masanori Yamauchi, “it is with great pleasure that we confirm the first collisions in the SuperKEKB accelerator, and we are glad to celebrate the start of the Belle II experiment after more than 7 years of improvements. I am eager to see how the results of Belle II will help us understand the nature of the Universe. We give thanks to all those who have supported this project. Regardless of any difficulties which may eventually arise before SuperKEKB reaches the planned luminosity, 40 times the record marked by KEKB, we will uphold our efforts to succeed in this, in collaboration with researchers from the whole World”.

Tom Browder, professor of the University of Hawaii and spokesperson of the Belle II team, declared that “after more than 7 years of construction and preparations by numerous researchers, engineers and by talented, dedicated students, the Belle II experiment has begun. It is a really rewarding moment for us to be part of this international collaboration. We now eagerly await for the start of the research programme of the first super-factory of B mesons [producing them] from electrons and positrons”.

Participants of SuperKEKB celebrating this collective success.

Carlos Mariñas, doctorate from the Institute of Corpuscular Physics (IFIC, CSIC-Universitat de València) is currently in the University of Bonn as an assistant coordinator for the operation of Belle II. “Detecting the first collisions is a great breakthrough for the teams involved, during the process of beam fine-tuning of the previous months. The experience of the Japanese particle acceleration scientists has led us to this point in a very short time, allowing to progressively turn Belle II on, without any risks to the experiment. Now, getting the most out of this exceptional machine rests on the hands of all of us, physicists working with the detector. And we accept the challenge indeed.”